1. Field of the Invention
The present invention relates to an image bearing member for use in electrophotography.
2. Description of the Background Art
An image forming method using an image bearing member used in photocopiers, facsimile machines, laser printers, direct digital plate makers, etc., includes at least the following processes: charging the image bearing member; irradiating the image bearing member with light to form a latent electrostatic image; developing the latent image with toner to obtain a visual toner image; transferring the toner image to an image carrying body (a transfer medium, typically paper); fixing the toner image; and cleaning the surface of the image bearing member.
Typically, known image bearing members for use in electrophotography have a photoconductive layer mainly made of selenium or an alloy thereof on an electroconductive substrate, or use inorganic photoconductive material such as zinc oxide and cadmium sulfide dispersed in a binder resin or an amorphous silicon-based material. In recent years, organic photoreceptors have come to be widely used because of their inexpensiveness, superior photoreceptor design flexibility, environmental cleanliness, etc.
Organic photoreceptors are classified into several general types: photoconductive resin, typified by polyvinylcarbazole (PVK); charge transfer complex typified by PVK-TNF (2,4,7-trinitrofluorenone); pigment dispersion typified by phthalocyanine-binder; and function separation using a combination of a charge generation material and a charge transport material. Among these, image bearing members of the function separation type are the most appealing.
The mechanism of electrostatic image formation in a function separation type photoreceptor (image bearing member) involves charging the image bearing member and then irradiating the image bearing member with light. The light passes through a transparent charge transport layer and is absorbed by a charge generation material in a charge generation layer, with the charge generation material that has absorbed light generating a charge carrier. The charge carrier is infused into the charge transport layer and is transported through the charge transport layer by an electric field generated by the charging, neutralizing the charge on the surface of the image bearing member to form a latent electrostatic image. In the function separation type image bearing member, a combination of a charge transport material absorbing mainly the ultraviolet and a charge generation material absorbing mainly visible light is known to be useful.
Most of the charge transport materials developed for the organic photoreceptor for use in electrophotography are low molecular weight compounds. Since such low molecular weight compounds do not have a layer forming property independently, these are typically dispersed and/or mixed in an inert polymer. Various charge transport materials have been developed.
Specific examples thereof include, but are not limited to, α-phenylstilbene derivative (as in Japanese patent application publication no. S58-190953 (JP-S58-190953-A), triphenylamine derivative (JP-H03-285960-A), and benzidine derivative (Japanese patent examined publication no. S58-32372 (JP-S58-32372-B). However, these charge transport materials are highly crystalline, thereby failing to impart sufficient solvent cracking resistance to the image bearing member.
At the same time, various methods have been proposed to prevent solvent cracking of the image bearing member. For example, JP-H04-368954-A describes adding a silicon-based graft polymer to the charge transport layer. JP-H04-368956-A describes setting the viscosity average molecular weight of a polycarbonate of from 2.5×104 to 15×104. JP-H07-128877-A describes setting the amount of residual solvent in the charge transport layer at 30 to 500 ppm.
These approaches are successful to some degree in preventing solvent cracking. However, these are combinations of a binder resin and a solvent and have no impact on the crystalline property of the charge transport material, which causes a problem of maintaining the anti-solvent cracking property.